The present invention relates to a dielectric ceramic composition for use in microwave communication and broadcasting equipment.
Recently, a dielectric material which has a large dielectric constant, a small dielectric loss, and a small value of temperature coefficient for resonant frequency (hereunder referred to as .tau..sub.f) in microwave ranges has been used as a resonator in satellite broadcasting receivers and car telephones. Conventional dielectric ceramics for use in such applications include BaO-TiO.sub.2 system ceramics, BaO-CaO-TiO.sub.2 system ceramics, and the like.
Since the dimensions of a resonator are proportional to 1/(.epsilon..sub.r).sup.1/2 (.epsilon..sub.r is the specific inductive capacity of a dielectric material), it is possible to obtain a very small resonator by employing a material with a high value of .epsilon..sub.r. For a BaO-TiO.sub.2 or a MgO-CaO-TiO.sub.2 system ceramic, the value of .epsilon..sub.r is 20-40. When a resonator made of these ceramics is used at the low-frequency end of the microwave range (close to 1 GHz), the disadvantage arises from the fact that the dimensions of the resonator become large.
In order to satisfy a high demand for dielectric resonators which are operated at a frequency close to 1 GHz, it is necessary to use dielectric ceramic compositions which have a much higher dielectric constant. In addition, these resonators must have a high value of Q (Q is the reciprocal of dielectric loss, tan.delta., i.e., Q=1/tan.delta.) and a low value of .tau..sub.f.
Japanese Laid-Open Specification No. 134562/1981 discloses a dielectric ceramic composition of the BaO-TiO.sub.2 -Nd.sub.2 O.sub.3 system which has a large .epsilon..sub.r (&gt;70) and a high Q (&gt;2000), but its .tau..sub.f is greater than 150 ppm/K which is not acceptable to practical use.
Japanese Laid-Open Specification No. 82501/1981 discloses a dielectric ceramic composition consisting of BaO-TiO.sub.2 -Nd.sub.2 O.sub.3, oxide of rare earth metals, and optionally PbO, which has a large .epsilon..sub.r of 91, a high Q of 4200 (at 2GHz), and a small .tau..sub.f of 5 ppm/K when PbO is present. When it does not contain PbO, however, it has a small .epsilon..sub.r of about 50.
When the composition contains PbO, PbO evaporates during firing, and it is difficult to obtain a stable composition. Furthermore, the evaporated PbO may contaminate an apparatus such as a firing furnace, and cause difficulties to the subsequent manufacture of other ceramic compositions using the apparatus.
Another known dielectric ceramic composition which is free from PbO is the BaO-TiO.sub.2 -Nd.sub.2 O.sub.3 -Sm.sub.2 O.sub.3 system. Since the system must have a high content of expensive Sm.sub.2 O.sub.3 in order to obtain a small .tau..sub.f, the resultant ceramic composition is quite expensive.